One of the promising control methods for a wind power generator system is a variable-speed and variable-pitch control method, in which the rotational speed of the windmill rotor (that is, the rotational speed of the generator) is variable and the pitch angle of the blades is variable. Advantages of the variable-speed and variable-pitch control method include increased energy capture from the wind and decreased output fluctuation.
With respect to the variable-speed and variable-pitch control method, it is of importance to optimize the control of the output power of the generator and the pitch angle of the blades. Japanese Translation of International Publication No. 2001-512804 discloses a control method in which the torque of the generator is controlled with a field orientation control while the pitch angle is controlled independently of the torque of the generator. In the disclosed control method, desired output power of the generator is determined in response to the rotational speed of the generator using a lookup table, and a torque command for the generator is determined from the desired output power. The torque of the generator is controlled by a field orientation control in response to the torque command. On the other hand, the pitch angle of the blades is controlled by PID control, PI control or PD control responsive to the deviation between the rotational speed of the generator and a desired rotational speed.
One issue for the wind power generator system is how to deal with occurrence of a transient wind null, that is, a short-time reduction in the wind speed. Generally, a wind power generator system is designed to generate rated power in the case where the rotational speed of the windmill rotor is equal to or larger than a rated rotational speed. In such a wind power generator system, the output power is reduced below the rated power, when the rotational speed of the windmill rotor is reduced below the rated rotational speed due to the occurrence of the transient wind null. This causes output power fluctuation and generation efficiency reduction.